This invention relates generally to float matresses and more particularly to a noninflatable, stable float mattress.
Float mattresses for floating a person in a pool, on a lake, or down a river have been in use for years. These matresses are basically of two types: inflatable air mattresses that obtain their buoyancy from air trapped within an outer bag and noninflatable mattresses that obtain their buoyancy from their material, such as cork, wood, or cellular polystyrene.
Both types of mattresses have advantages and drawbacks. Inflatable mattresses are light, portable, and compact but are easily damaged by punctures and must be continually repaired. Noninflatable mattresses, on the other hand, are more durable, but they are relatively heavy and cumbersome to transport.
A conventional mattress of either type, however, suffers from a more serious drawback. Because of its relatively large volume and low weight, a conventional mattress displaces litter water and floats with most of its volume above the waterline. With the mattress in its normal horizontal position, this causes a downward force acting on the mattress, its center of gravity, to be high relative to the upward opposing buyoyant force, its center of buoyancy. Such a relationship makes a mattress in water highly unstable and subject to capsizing by sudden shift of weight on the mattress or by rough water buffeting it about. As the mattress heels in response to the weight shift or rough water, its center of buoyancy shifts to a different position and the upward force strives to rotate the mattress about the center of gravity. So long as the metacenter of the mattress (the intersection of the normal center of buoyancy with the shifted center of buoyancy) is above the center of gravity, the mattress will right itself. But if the metacenter moves below the center of gravity, a situation likely to occur with the center of gravity high relative to the center of buoyancy, the mattress will capsize.
An example of a conventional, noninflatable matterss is shown in U.S. Pat. No. 4,275,473 to Poirier. The Poirier mattress comprises a plurality of rigid, parallel buoyant blocks fixed to an outer skin. The blocks are of a low density material with a large surface area, allowing the mattress to be easily upset by the shifting of a person's weight on the mattress or by rough water tipping the mattress. U.S. Pat. No. 3,380,088 to d'Adesky is of a similar design and suffers from the same drawback.
To achieve better stability, a float mattress should displace as little water as possible to raise its center of buoyancy and yet have a center of gravity as low as possible. U.S. Pat. No. 3,369,262 to Judd shows a flotation cushion with a relatively low center of gravity. The cushion absorbs water rather than displaces it. This absorption lowers the cushion's center of gravity, but the absorbed water is not easily removed and can capsize the cushion with a slight shift of a person's weight thereon. The saturated material within the cushion also makes the cushion heavy and therefore difficult to carry or dry.